In Slovenia, the current vaccination rate among the population is slightly above 30%, and even though the demand currently exceeds what is available, it is considered that this could be reversed by the end of summer. Many Slovenians are hesitant to get vaccinated, among other reasons because of a concern about rare adverse effects of vaccines, mostly thrombotic events. This paper will look into the currently known data on such events and answer the most frequently asked questions.

What are the post-vaccination thrombotic events?

A rare type of blood clots, called *vaccine-induced thrombotic thrombocytopaenia*, has been reported for vector-type vaccines (among them AstraZeneca and Janssen so far available in Slovenia). Such blood clotting develops with a simultaneously reduced thrombocytes count, which initiates the clotting process. They develop 4–28 days after vaccination.

Such thromboembolisms are different from usual blood clots. They occur at atypical sites such as the cerebral sinuses and spleen. Very rarely, they can develop in the arterial circulation.

Is thrombotic thrombocytopaenia life-threatening?

It is an emergency condition, and its danger depends on its site; we are primarily concerned about the brain. The estimated mortality rate in early reports was 20%, but it decreased to 1–2% due to the current awareness of healthcare professionals in Europe.

This is thanks to the effective treatment and fast diagnosis, which is relatively simple for such complications.

How common is this complication?

Thrombotic events following vaccination are very rare, approximately 1/100,000. This means that with a 100% vaccination rate in Slovenia, 20 such events can be expected. The complication mostly develops in women below the age of 60, but no convincing age limit has been demonstrated, and it can also develop in men.

Can I preventively take aspirin after vaccination?

Aspirin is not recommended to prevent such complications. That is because the mechanism is completely different than for the standard, non-immune mediated clots. Aspirin may even cause additional harm since it thins the blood and, in the case of thromboembolism, further increases the likelihood of bleeding.

Individuals already receiving anticoagulant therapy should not interrupt it because of vaccination.

Is vaccination not recommended if I have ever had a blood clot?

There is currently no evidence indicating that a history of clots would increase the risk of such complications. Persons with a history of clots or with a close relative who has experienced clots can be normally vaccinated with any vaccine.

Such clots are immune-mediated, i.e. the mechanism of their formation is entirely different than for the usual clots. So far, no specific group of persons at particular risk of such complications has been identified.

Should I request an mRNA vaccine for the second dose of vaccine?

There has been no case of a thromboembolic complication developing after the second dose. At the same time, the effectiveness of the so-called crossover vaccination has not been officially proven, although the initial results are promising. Therefore, there is no point in switching the vaccine for the second dose. In the case of the Janssen vaccine, a second dose is not necessary.

What are the signs of a post-vaccination thrombotic event?

Some symptoms after vaccination are expected, such as pain at the injection site, fever, chills, malaise … These symptoms represent an active immune system responding to a foreign body. The signs of a thromboembolic event are more specific. People should mostly pay attention to:

  • severe headache, especially if accompanied by blurred vision
  • difficulties breathing accompanied by chest pain
  • severe abdominal pain
  • a painful or reddened limb
  • pallor and coldness of a limb

If you notice any of these symptoms, seek emergency medical assistance.

What can be expected in the future?

Promising studies are underway investigating the mechanism of the occurrence of such clots and the possibilities to prevent them. It can be expected that the vaccines will keep on improving and that such rare complication will be resolved.

The frequency of such adverse effects is nevertheless so low that vaccination is recommended to the entire population. Vector-type vaccines are very safe and almost 100% effective in preventing a severe clinical course of the disease, including a coagulation disorder. It is deemed that during an average course of the infection, the risk of blood clotting is 10-times higher than with vaccination, and if the course of the disease is severe, the risk is further increased.

Thus, the benefits of vaccination undoubtedly outweigh the risks. It is, however, important to pay attention to our health condition for a month after the first vaccination and see the doctor in due time if any suspicious symptoms develop.